How to get Timestamp of a Column?

cqlsh:ks> // Create table with Primary Key
cqlsh:ks> drop table student;
cqlsh:ks> create table student(
      ...    name text,
      ...    class text,
      ...    primary key(name));
cqlsh:ks>
cqlsh:ks> // Insert Data
cqlsh:ks> insert into student(name, class) values('Bob', '1st');
cqlsh:ks>
cqlsh:ks> // Retrieve Timestamp for a Column #63
cqlsh:ks> select name, class, writetime(class) from student;

 name | class | writetime(class)
------+-------+------------------
  Bob |   1st | 1476811810122259

(1 rows)
cqlsh:ks>
cqlsh:ks> // Timestamp cannot be retrieved for Primary Key
cqlsh:ks> select name, writetime(name) from student;
InvalidRequest: Error from server: code=2200 [Invalid query] message="Cannot use selection function writeTime on PRIMARY KEY part name"